drm/i915: Replace nested subclassing with explicit subclasses

In the next patch, we will want a third distinct class of timeline that
may overlap with the current pair of client and engine timeline classes.
Rather than use the ad hoc markup of SINGLE_DEPTH_NESTING, initialise
the different timeline classes with an explicit subclass.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706210710.16251-1-chris@chris-wilson.co.uk